Tuesday, 27 March 2012

Neurons
are a lot like electrical wires. In fact, the axons are electrical
wires. As you’ve no doubt noticed, electrical wires are fast. As I type, words
pass through my computer to appear on my monitor the instant my fingers press
down the keys. That’s pretty darn fast. Your brain has to be
fast, too. If you encounter a slobbering lion, your eyes need to tell your
brain to tell your muscles to run, now, or be tasty lion lunch. If you
put your hand on something hot, you need to pull it off before you burn.

Run!

The
point is, your brain has to be fast. And what’s one of the fastest possible
ways to transmit information? Electricity.

To learn how neurons do
this, we first have to talk a little more about the structure of neurons.
Because even though neurons act like electrical wires, they aren’t made of
metal. Let’s zoom in on a section of the axon:

(Side
note: if you don’t know what I mean by “axon” and “soma”, check out the post on neurons.)

Every neuron in your
body is surrounded by a cell membrane, which separates it from the outside
world, just like your skin does for you. The cell membrane is made of fats
called lipids (the yellow circles with two tails). Under normal conditions, there
is an electrical charge across the cell membrane. Have you ever rubbed your
feet along a carpet, touched a metal doorknob, and felt an electric shock? This
happens because your body has a different electrical charge than the doorknob.
The neuron is the same way. The inside has a negative charge, and the outside
has a positive charge. This happens because of ions, which are atoms that carry
a positive or negative charge. There are mostly negative ions inside the cell,
and mostly positive ions outside the cell. The important ions for action
potentials are potassium (K+) and sodium (Na+), which are both positively
charged. Let’s add that into the picture: (The purple (-) symbols are negative ions)

Under
normal circumstances, the ions are stuck where they are, because they can’t
cross the fatty cell membrane. See the red and blue things in the picture?
These are ion channels. They’re like doors across the membrane that only ions
can use, but they’re almost always closed.

Now,
let’s say something happens. Maybe you step on something sharp. Maybe you see
two people fiercely sword fighting on top of a red hot air balloon. Maybe your
friend ate too many beans and now something smells awful and you need to get
out of the room or suffocate. The point is, there’s a stimulus. Something
important that your neurons can respond to. The stimulus sets events in motion.

The
stimulus does this by opening one of those ion channels. In our picture, the
stimulus opens the blue ion channel, which is the closest channel to the soma.

Have you ever taken two
magnets and tried to stick the two positive ends together? Or the two negative
ends? It doesn’t work. No matter how hard you try, you can’t get those magnets
to stick. This is because ions of the same charge don’t like to be around each
other. But if you flip one magnet around, so the positive side is facing the negative side, they stick fast. Like repels like, and opposites attract, at least where ions are
concerned. So once the blue channel opens, all the positive sodium ions outside
the axon go rushing away from all the other positive ions and into the
negatively charged axon. As a result, the electrical charge inside the cell
becomes more positive. Here’s a picture of that:

Now it’s time for the red channels
to get involved. These channels are voltage-gated. When they sense the change
in membrane potential (that is, when the inside of the axon starts to get more
positive) they open, allowing even more positive ions to flow in. This starts up the action potential. Now the electrical current is going to race down the axon faster than a race car on a speedway. Here’s a
picture of that:

As
each section of the axon gets more positive, more and more voltage-gated
channels open, each one further from the blue channel than the one before. Once
it starts, it doesn’t stop until the positively charged message travels down
the whole axon.

Eventually, the action potential does stop. When the cell gets too positive, other voltage-gated channels
open, which allow potassium to leave the axon, making it negatively charged
again. There’s also pumps (the green things) which dump out all that excess
sodium and maintain the neuron’s normal, negative state. Once the membrane
potential is reset, a new action potential can fire the next time you see
something interesting (like the hungry lion’s hungry sister).

Action
potentials range in speed from 1 meter per second to 100 meters per second. 100
meters is the same as 328 feet, which is longer than a football field. That means
that in one measly little second, an action potential can travel farther than an
entire football field. Since you are a whole lot smaller than a football field,
this means that things in your body can happen really, really fast.

Once the action potential
is complete, the message needs to be transmitted to the dendrites of the next
cell in line. This happens at the synapse. A post on this is coming soon.

Monday, 19 March 2012

Neurons are the basic building blocks of the brain, and in future posts I’ll be referring to them over and over again. Neurons control almost everything you do. There’s 86 billion of them in your brain alone. That’s more than 12 times the number of people on the entire planet. If you took all the neurons in the brain at stretched them out, you’d get a line 860 kilometers long, or about 534 miles. That’s longer than the distance from San Francisco, California to Tijuana, Mexico, and would take about 10 hours to drive in a car. But because neurons are so small that you can only see them through a microscope, the line would be invisible! There’s even more neurons running through your body, controlling your heart rate and breathing and digestion and every little movement your muscles make. They help you see and smell and hear and they know when you’re hungry and thirsty. When you think, there’s an electrical storm in your brain as millions of neurons share pictures and words and feelings. Your neurons are, quite simply, you.
So what is a neuron, exactly? A neuron is a type of cell. Your body is made up of cells, 10 trillion of them! That over 100 brains’ worth of cells. Neurons are a type of special type of cell that send, receive, and store electrical and chemical information. A neuron has three parts:

A neuron. The neurons in the visual, motor, memory and language centers of my brain, as well as in my eyes and muscles, all had to talk to each other so that I could draw this picture.

THE SOMA:
The soma is the neuron’s control center. It keeps the neuron healthy, produces all the proteins and chemicals that a neuron needs to function, and holds the neuron’s DNA.

THE AXON:
The axon sends information to other neurons. It can be extremely long. You have an axon that goes from your spinal cord all the way to the tip of your big toe. The axon conducts information much like an electrical wire. (More on this in a future post.)

THE DENDRITE:
The dendrite receives information from other neurons. It joins up with another neuron’s axon at a junction called a synapse, which is a whole big future blog post in itself. Dendrites are shorter than axons, and highly branched. They look a lot like trees, which is why we often call them “dendritic arbors.” (“Arbor” means “tree”.)

When one neuron has an important message to send (like, “Ow-wow! Get your hand off that hot plate!”), it sends the message as an electrical signal down its axon. This message is called an action potential. The dendrite of the next neuron in line (I’ll call it neuron #2) picks up the signal and sends it to its soma. If the signal is strong enough, it triggers a new action potential in neuron #2’s axon, which signals to the dendrite of neuron #3, and so on until you pull your hand off the hot plate.
That’s Neurons 101. Coming up are how the action potential works, what’s a synapse, and more about how your muscles respond to that hot plate.